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Abstract:

To improve jump characteristic of
BaTiO3--(Bi1/2Na1/2)TiO3 material.
There is provided a process for producing a semiconductive porcelain
composition in which a part of Ba is substituted with Bi--Na, the process
including a step of preparing a (BaQ)TiO3 calcined powder (in which
Q is a semiconductor dopant), a step of preparing a (BiNa)TiO3
calcined powder, a step of mixing the (BaQ)TiO3 calcined powder and
the (BiNa)TiO3 calcined powder, a step of molding and sintering the
mixed calcined powder, and a step of heat-treating the obtained sintered
body at 600° C. or lower; and a PCT heater employing the element
prepared by the above steps.

Claims:

1. A process for producing a semiconductive porcelain composition in which
a part of Ba is substituted with Bi--Na, the process comprising a step of
heat-treating the semiconductive porcelain composition to which an
electrode is not formed at 600.degree. C. or lower in an atmosphere
containing oxygen.

2. (canceled)

3. A process for producing a semiconductive porcelain composition in which
a part of Ba is substituted with Bi--Na, the process comprising a step of
heat-treating the semiconductive porcelain composition to which an
electrode is not formed at 600.degree. C. or lower in the air.

4. A process for producing a semiconductive porcelain composition in which
a part of Ba is substituted with Bi--Na, which is a process for producing
a semiconductive porcelain composition provided with an electrode, the
process comprising:a step of forming the electrode to the semiconductive
porcelain composition, anda step of heat-treating the semiconductive
porcelain composition at 600.degree. C. or lower in the air.

5. A process for producing a semiconductive porcelain composition in which
a part of Ba is substituted with Bi--Na, the process comprising:a step of
preparing a (BaQ)TiO3 calcined powder (wherein Q is a semiconductor
dopant);a step of preparing a (BiNa)TiO3 calcined powder;a step of
mixing the (BaQ)TiO3 calcined powder and the (BiNa)TiO3
calcined powder;a step of molding and sintering the mixed calcined
powder; anda step of heat-treating said obtained sintered body at
600.degree. C. or lower in an atmosphere containing oxygen or in the air.

6. A heater comprising a heating element comprising a semiconductive
porcelain composition provided with an electrode, which is obtained by
forming the electrode to the semiconductive porcelain composition
obtained by the production process as claimed in claim 1.

7. A heater comprising a heating element comprising a semiconductive
porcelain composition provided with an electrode, which is obtained by
forming the electrode to the semiconductive porcelain composition
obtained by the production process as claimed in claim 3.

8. A heater comprising a heating element comprising a semiconductive
porcelain composition provided with an electrode, which is obtained by
the production process as claimed in claim 4.

9. A heater comprising a heating element comprising a semiconductive
porcelain composition provided with an electrode, which is obtained by
forming the electrode to the semiconductive porcelain composition
obtained by the production process as claimed in claim 5.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of PCT International Patent
Application No. PCT/JP2009/054809, filed Mar. 12, 2009, and Japanese
Patent Application No. 2008-071353, filed Mar. 19, 2008, in the Japanese
Patent Office, the disclosures of which are incorporated herein by
reference.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to a process for producing a
semiconductive porcelain composition having a positive temperature
coefficient of resistance (PTC), and a heater employing the
semiconductive porcelain composition.

[0004]2. Description of the Related Art

[0005]BaTiO3 semiconductive porcelain composition has been known as a
material showing a positive resistance temperature characteristic. When
SrTiO3 or PbTiO3 is added to the BaTiO3 semiconductive
porcelain composition, a Curie temperature can be shifted, but only
PbTiO3 is an additive material that enables shifting in a positive
direction. However, since PbTiO3 contains an element causing
environmental pollution, a material using no PbTiO3 as an additive
material has been desired. Consequently, a semiconductive porcelain
composition in which a part of Ba in BaTiO3 is substituted with
Bi--Na was proposed (see Patent Document 1).

[0006]Where a BaTiO3 material is treated by, for example, sintering
in a reducing atmosphere so as to decrease room temperature resistivity,
there is a problem that a temperature coefficient of resistance (jump
characteristic) is decreased. When the jump characteristic is decreased,
there is a problem that switching does not occur at the objective
temperature. Therefore, to improve the jump characteristic, it is
proposed to conduct a heat treatment at high temperature exceeding
1,100° C. (Patent Document 2).

[0007]Patent Document 1: WO 2006/106910

[0008]Patent Document 2: JP-A-56-169301

SUMMARY OF THE INVENTION

[0009]In recent years, PTC material is frequently used in high temperature
environment in view of improvement in heat resistance characteristic
thereof. However, further improvement in jump characteristic has been
desired to enable the material to be used in higher temperature
environment. BaTiO3--(Bi1/2Na1/2)TiO3 material, which
is free of Pb and in which a part of Ba is substituted with Bi--Na, has
sufficient jump characteristic by itself, but it is necessary to improve
the jump characteristic in view of the above-mentioned demand. Although
the above-described heat treatment may be considered to improve the jump
characteristic thereof, it could be confirmed that even though the heat
treatment as applied to a BaTiO3 material containing Pb is merely
applied as it is, the jump characteristic is not improved.

[0010]Aspects of the present invention has been made in view of the above
circumstances, and has an object to improve a jump characteristic of a
semiconductive porcelain composition in which a part of Ba of BaTiO3
in BaTiO3--(Bi1/2Na1/2)TiO3 material and the like is
substituted with Bi--Na.

[0011]As a result that the present inventors have conducted heat treatment
to a BaTiO3--(Bi1/2Na1/2)TiO3 material, it was
confirmed that PTC characteristics are impaired by high heat temperature
treatment such that an element becomes an insulator at the treatment
temperature of 1,280° C. The reason therefor is considered that
trivalent Ti formed by valence control is oxidized into tetravalent Ti,
whereby a carrier is reduced.

[0012]Furthermore, since the jump characteristic of a BaTiO3 material
containing Pb depends on an oxygen amount of grain boundary, it is
necessary to introduce oxygen into the grain boundary after reducing
oxygen defect caused during sintering. Therefore, unless the heat
treatment is conducted at 800° C. or higher that recovers oxygen
defect, the practicable jump characteristic cannot be obtained. On the
other hand, in the BaTiO3--(Bi1/2Na1/2)TiO3 material,
not only oxygen amount of grain boundary but also component distribution
of a material affect the jump characteristic. Therefore, it is possible
to improve the jump characteristic only by introducing oxygen into the
grain boundary without restoring oxygen defect. Consequently, it has been
found that the jump characteristic can be improved even when heat
treatment is conducted at 600° C. or lower.

[0013]Based on the above finding, aspects of the present invention are to
improve a jump characteristic by heat-treating a semiconductive porcelain
composition in which a part of Ba is replaced with Bi--Na at 600°
C. or lower. The heat treatment may be conducted in the air, but is
preferably conducted in an atmosphere containing oxygen. More quickly,
heat treatment in an oxygen atmosphere is preferred.

[0014]Furthermore, aspects of the present invention are to improve a jump
characteristic by forming an electrode to a semiconductive porcelain
composition in which a part of Ba is substituted with Bi--Na, followed by
heat-treating the same at 600° C. or lower in the air. In the case
of forming an electrode, heat treatment in the air is preferred in order
to avoid deterioration of the electrode.

[0015]The semiconductive porcelain composition produced by aspects of the
present invention has a jump characteristic that is not appeared in the
conventional semiconductive porcelain composition in which a part of Ba
of BaTiO3 is substituted with Bi--Na. Therefore, a heater employing
a heating element comprising the semiconductive porcelain composition
produced by aspects of the present invention is suitable for use in
higher temperature environment.

[0016]According to aspects of the present invention, a jump characteristic
of a semiconductive porcelain composition in which a part of Ba is
substituted with Bi--Na can be improved.

[0017]Additional aspects and/or advantages of the invention will be set
forth in part in the description which follows and, in part, will be
obvious from the description, or may be learned by practice of the
invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]These and/or other aspects and advantages of the invention will
become apparent and more readily appreciated from the following
description of the embodiments, taken in conjunction with the
accompanying drawings of which:

[0019]FIG. 1 is a schematic view showing the constitution of a heater
element; and

[0020]FIG. 2 includes graphs showing change in voltage and current with
respect to temperature of a heater element.

[0025]Reference will now be made in detail to the present embodiments of
the present invention, examples of which are illustrated in the
accompanying drawings, wherein like reference numerals refer to the like
elements throughout. The embodiments are described below in order to
explain the present invention by referring to the figures.

[0026]Aspects of the present invention include heat-treating a
BaTiO3--(Bi1/2Na1/2)TiO3 material in which a part of
Ba is substituted with Bi--Na at 600° C. or lower. In the case of
exceeding 600° C., the
BaTiO3--(Bi1/2Na1/2)TiO3 material gradually exhibits
insulating characteristic and converts to an insulating material at
1,280° C. Accordingly, 600° C. that is a range having
practically no problem is employed as the upper limit of the heat
treatment temperature. Regarding heat treatment time, since a jump
characteristic is saturated when the heat treatment time is too long, the
heat treatment time is preferably about 12 hours. Incidentally, the
effect can be obtained even in the vicinity of room temperature if the
heat treatment is conducted for a long period of time. The heat treatment
in nitrogen decreases a jump characteristic and is therefore not
preferred.

[0027]According to aspects of the present invention, the step of preparing
a (BaQ)TiO3 calcined powder (Q is a semiconductor dopant) is such
that BaCO3, TiO2 and a raw material powder of semiconductor
dopant, such as La2O3 or Nb2O5, are firstly mixed to
prepare a raw material powder, and it is then calcined. Calcination
temperature is preferably a range of from 600° C. to 1,000°
C., and calcination time is preferably 0.5 hour or more. When the
calcination temperature is lower than 600° C. or the calcination
time is shorter than 0.5 hour, (BaQ)TiO3 is hardly formed, and
unreacted BaCO3, BaO and TiO2 disturbs a uniform reaction with
(BiNa)TiO3, and prevents development of PTC characteristics, which
is not preferred. When the calcination temperature exceeds 1,000°
C., the effect of controlling Bi volatilization is lost, and stable
formation of (BaQ)TiO3--(BiNa)TiO3 is prevented, which is not
preferred.

[0028]According to aspects of the present invention, the step of preparing
a (BiNa)TiO3 calcined powder is such that Na2CO3,
Bi2O3 and TiO2 as raw material powders are firstly dry
mixed to prepare a mixed raw material powder, and it is then calcined.
Calcination temperature is preferably a range of from 700° C. to
950° C., and calcination time is preferably from 0.5 hour to 10
hours. When the calcination temperature is lower than 700° C. or
the calcination time is shorter than 0.5 hour, unreacted NaO reacts with
ambient moisture or a solvent in the case of wet mixing, causing
compositional deviation and variation of characteristics, which is not
preferred. When the calcination temperature exceeds 950° C. or the
calcination time exceeds 10 hours, Bi volatilization proceeds to cause
compositional deviation, thereby formation of a secondary phase is
accelerated, which is not preferred.

[0029]According to aspects of the present invention, the step of mixing
the (BaQ)TiO3 calcined powder and the (BiNa)TiO3 calcined
powder is such that these calcined powders are blended in given amounts,
followed by mixing. Mixing may be either of wet drying using pure water
or ethanol and dry mixing, and the dry mixing is preferably conducted
because compositional deviation can further be prevented. Depending on a
particle size of a calcined powder, pulverization may be conducted after
mixing, or mixing and pulverization may simultaneously be conducted. An
average grain size of the mixed calcined powder after mixing and
pulverization is preferably from 0.6 μm to 1.5 μm.

[0030]In the above step, when Si oxide is added in an amount of 3.0 mol %
or less, or Ca carbonate or Ca oxide is added in an amount of 4.0 mol %
or less, the Si oxide can suppress abnormal growth of crystal grains and
additionally facilitates to control resistivity, and the Ca carbonate or
the Ca oxide can improve sinterability at low temperature, which are
preferred. Where either one of them is added in an amount exceeding the
above limited amount, a composition does not show semiconductive
property, which is not preferred. The addition is preferably conducted
before mixing in each step.

[0031]According to aspects of the present invention, the step of molding
and sintering a calcined powder obtained by mixing the (BaQ)TiO3
calcined powder and the (BiNa)TiO3 calcined powder is such that the
mixed calcined powder is firstly molded by the desired molding means. As
necessary, the pulverized powder may be granulated by a granulating
apparatus before molding. Density of a molded article after molding is
preferably from 2 to 3 g/cm3. The sintering can be conducted in the
air, a reducing atmosphere or an inert gas atmosphere of low oxygen
concentration at a sintering temperature of from 1,200° C. to
1,400° C. for a sintering time of from 2 hours to 6 hours. In the
case that granulation is conducted before mixing, a binder-removal
treatment is preferably conducted at from 300° C. to 700°
C. before sintering.

[0032]According to aspects of the present invention, the step of forming
an electrode to a semiconductive porcelain composition is such that a
sintered body is processed into a plate shape to prepare a test piece,
and an ohmic electrode is then formed on the surface thereof. Ti, Cr, Ni,
Al, Fe, Cu, Ag--Zn and the like can be selected as the ohmic electrode.
The ohmic electrode may be formed by baking onto the test piece, or by
sputtering or vapor deposition. The ohmic electrode is preferably covered
with a covering electrode, for example, Ag, Al, Au or Pt, to protect the
ohmic electrode.

Example 1

[0033]Raw material powders of BaCO3 and TiO2 as main materials
and La2O3 as a semiconductor dopant were prepared, and were
blended so as to be (Ba0.994La0.006)TiO3, and as
necessary, CaCO3 and SiO2 were further added as sintering aids,
and were mixed in ethanol. The mixed raw material powder obtained was
calcined at 900° C. for 4 hours in the air to prepare a
(BaLa)TiO3 calcined powder.

[0034]Raw material powders of Na2CO3, Bi2O3 and
TiO2 were prepared, and were blended so as to be
(Bi0.5Na0.5)TiO3. As necessary, a sintering aid was
further added, followed by mixing in the air or ethanol. The mixed raw
material powder obtained was calcined at 800° C. for 4 hours in
the air to prepare a (BiNa)TiO3 calcined powder.

[0035]The (BaLa)TiO3 calcined powder and the (BiNa)TiO3 calcined
powder were blended so as to be
[(Bi0.5Na0.5)0.1(Ba0.994La0.006)0.9]TiO.sub-
.3, and were mixed and pulverized by a pot mill using pure water as a
medium until a mixed calcined powder became 1.0 μm to 2.0 μm,
followed by drying. PVA was added to a pulverized powder of the mixed
calcined powder, and the resulting mixture was mixed and then granulated
by a granulating apparatus. The granulated powder obtained was molded
with a single-screw press apparatus, and a molded article obtained was
subjected to binder removal at 700° C., and then sintered in
nitrogen at a sintering temperature of 1,340° C. for 4 hours,
thereby obtaining a sintered body. Incidentally, when the (BaLa)TiO3
calcination temperature is 900° C. or lower, BaCO3 and
TiO2 can remain in a calcined powder, or when BaCO3 and
TiO2 are post-added to a calcined powder in which the
(BaLa)TiO3 calcination temperature is 1,000° C. or higher and
1,200° C. or lower, the characteristics can be stabilized.

[0036]The sintered body obtained was processed into a plate shape of 10
mm×10 mm×1 mm to prepare a test piece, and an ohmic electrode
constituted of Ag--Zn and a covering electrode comprising Ag as a main
component to be formed thereon were simultaneously baked onto the test
piece to prepare a test element. Temperature change of resistivity of the
test piece was thereafter measured in a range of from room temperature to
270° C. with a resistance measuring instrument, and room
temperature resistivity, Curie temperature and temperature coefficient of
resistance (InR1-InRc)×100/(T1-Tc) in which
R1: maximum resistivity, Rc: resistivity at Tc, T1:
temperature showing R1, and Tc: Curie temperature, were
measured as PTC characteristics. After the measurement, the electrode was
removed, and heat treatment was conducted at from room temperature to
800° C. After the heat treatment, an electrode was again formed on
the material, and the above PTC characteristics were evaluated.

[0037]Table 1 shows PTC characteristics when heat treatment was conducted
at from 20° C. to 600° C. in an oxygen atmosphere, and
Table 2 shows PTC characteristics when heat treatment was conducted at
from 20° C. to 800° C. in the air. Focusing on the
temperature coefficient of resistance, it could be confirmed that the
samples in which the heat treatment was conducted in an oxygen atmosphere
and in the air all show a value higher than that of the temperature
coefficient of resistance of the samples before heat treatment.

[0038]PTC characteristics were measured in the same manner as in Example
1. After the measurement, heat treatment was conducted at from room
temperature to 800° C. in such a condition as it was without
removing the electrode. Since the electrode deteriorates when the heat
treatment is conducted in an oxygen atmosphere in a state of forming an
electrode and since no effect can be obtained when the treatment is
conducted in nitrogen, the heat treatment was conducted in the air. After
the heat treatment, the PTC characteristics were evaluated.

[0039]Table 3 shows PTC characteristics when heat treatment was conducted
from 20° C. to 800° C. in the atmosphere. Focusing on the
temperature coefficient of resistance, it could be confirmed that the
samples in which the heat treatment was conducted in the air all show a
value higher than that of the temperature coefficient of resistance
before the heat treatment, up to 600° C. However, it was confirmed
that the value lower than that of the temperature coefficient of
resistance before the heat treatment is shown at 800° C.

Example 3

[0040]An electrode was formed on the edge face of each of a material
heat-treated at 400° C. and a material which was not heat-treated,
to prepare a heater element. FIG. 1 shows the constitution of a heater
element, and the heater element is constituted by sandwiching a heater
element 15 between a pair of casing 13 (13a, 13b) equipped with a
radiation fin 11. Voltage was applied to the heater element 15 through a
feed terminal 17a provided in one casing 13a and a feed terminal 17b
provided in the other casing 13b, thereby inducing heat generation of the
heater element 15. After arranging the prepared heater element in a
thermostat bath and increasing temperature to a given temperature,
voltage of 13V was applied to the feed element, changes in voltage and
current with respect to temperature were observed (see Table 4).

[0041]FIG. 2 includes views showing change in voltage and current
(vertical axes) with respect to temperature (horizontal axis). It is seen
that, as compared with non-heat treatment ((a) in the Figure),
characteristics after heat treatment ((b) in the Figure) are such that a
current value is extremely low at high temperature side and safety at
high temperature is improved.

[0042]Although aspects of the present invention have been described in
detail and by reference to the specific embodiments, it is apparent to
one skilled in the art that various modifications or changes can be made
without departing the spirit and scope of the present invention.

[0043]As described above, the semiconductive porcelain composition
according to aspects of the present invention has jump characteristic
which is not appeared in the conventional semiconductive porcelain
composition in which a part Ba of BaTiO3 is substituted with Bi--Na.
A heater employing a heating element comprising the semiconductive
porcelain composition is suitable for use in higher temperature
environment.